It is often desirable to provide a controlled phase shift with respect to a signal. For example, radio frequency (RF) circuits often utilize 90° phase differentials with respect to a signal, e.g., in-phase (I) and quadrature (Q) signal components, to provide desired signal processing. One circuit using such 90° phase differentials is a single sideband mixer, typically using 2 90° phase shifters in order provide I and Q components with respect to a carrier signal and I and Q components with respect to a local oscillator (LO) signal. Through proper phase shifting and combining of the signals (destructive combining of undesired frequencies e.g., sidebands and/or images), such a single sideband mixer can be utilized to cancel undesired frequencies.
To suppress undesired frequencies through destructive combining, the 90° phase differentials utilized by the single sideband mixer should be very accurate. Likewise, the amplitudes of the signal components for which destructive combining is desired should be very close.
The aforementioned 90° phase differentials have often been provided using a polyphase filter circuit. Such circuits are typically embodied as a resistor and capacitor (RC) network, where the number of stages of resistors and capacitors determines the accuracy of the phase shift provided. However, there is parasitic capacitance associated with every node in such a circuit. At high frequencies, parasitic capacitance will cause attenuation of an RF signal as it passes through the network. For example, for a polyphase filter operating with an RF signal having a characteristic frequency of about 2 GHz, 7 dB of attenuation is not uncommon.
Moreover, the amount of parasitic capacitance and transmission delays associated with various signal paths within the polyphase filter must be carefully matched in order to provide an accurate phase shift output. Such matching concerns have typically been addressed in the prior art by providing excess signal path portions, thereby resulting in a complex and large polyphase filter structure.
Accordingly, a need exists in the art for polyphase filters adapted to provide a desired phase shift with a minimal amount of signal attenuation. Moreover, there is a need in the art for the structure of such polyphase filters to be compact.